The present invention relates to a network system diagnosis method, and in particular, to a diagnosis method of a star ring network system in which a plurality of line concentrators are connected through a pair of transmission lines in a form of a ring or a loop with signal transmission directions of the lines set to be opposite to each other and a plurality of stations are linked to each line concentrator in a form of a star or in a radial form.
In a ring network system, a plurality of line concentrators are connected via a first transmission line and a second transmission line, the first and second transmission lines having the opposite transmission directions, and a plurality of terminal stations, ST's are linked to each line concentrator. Signal transmission between ST's is ordinarily conducted via the first ring transmission line, namely, the primary transmission line. The second ring transmission line is a subtransmission line to be set as a spare line or a standby line.
In a form of the ring network system, only the plurality of line concentrators are included in the ring transmission lines, and branch lines connecting each station to the respective line concentrator are disposed externally with respect to the ring, thereby preventing a failure transmission through the ring transmission lines. In a recent network system, on the other hand, as desribed in an article "Local Area Network in Token Ring System" (BIT, Kyoritsu Shuppan Sha, Vol. 16, No. 3, 1984) for example, a plurality of ports are disposed on a first ring transmission line in each line concentrator and a plurality of ST's are linked in a form of a star via branch lines to these ports so as to construct a star ring network. In this configuration, although the line concentrators and all ST's can be advantageously controlled by use of only one ring control protocol, there exists a disadvantage that when a failure occurs in a branch line or an ST associated with a line concentrator, the failure prevents the data transmission of the overall network system.
To prevent this difficulty, each line concentrator is provided with a control ST connected to a position furthest downstream in the first ring transmission line for controlling the configuration of transmission lines so that the control ST separates a failed port (to form a bypass) or establishes/releases a loopback path between two ring transmission lines.
According to "A Token Ring Architecture for Local Area Networks" published in "Contribution of Working Paper to IEEE Project 802 on Local Area Networks, Mar. 8, 1982, each ST constituting a network is provided with an information item (port number) for identifying a location of the ST in a line concentrator to which the ST is connected, and when an ST detects an abnormality of the network, the ST is caused to issue an abnormality notification frame containing the port number. In this case, the other ST's sequentially relay the abnormality notification frame from itself, the ST terminates the operation of the frame transmission. This provision allows only the ST at a location immediately downstream with respect to the failure position to continue issuing the abnormality notification (because the abnormality notification frame cannot be received from an ST placed at an upstream location), and the control ST checks the port number contained in the frame. If the ST issuing the frame is found to be under control of the control ST, a bypass operation is achieved for the pertinent port. If the failure occurrence point is in a branch line of the port or in an ST (first ST) connected thereto, the failure recovery operation is completed with the bypass operation.
In the case described above, if the failure is assumed to have occurred in a ring transmission line between line concentrators, the abnormal state cannot be restored even if the bypassing of the first ST is conducted, and a second ST positioned at a location immediately downstream with respect to the first ST continues sending the abnormality notification frame. The control ST checks the transition of the states before and after the bypass operation, namely, the port number of the port from which the abnormality notification frame has been issued so as to determine that the failure has occurred at a position outside the self-line concentrator, said position being on the upstream side in the first ring transmission line, and therefore restores the bypass port to the original state and establishes a loopback path between an output terminal of the second ring transmission line and an input terminal of the first ring transmission line. In this case, the second ring transmission line is interrupted; consequently, a control ST of a line concentrator adjacent to the failure position on the downstream side in the second transmission line (i.e. on the upstream side in the first transmission line) effects the failure recovery operation to establish a loopback line between the output terminal of the first ring transmission line and the input terminal of the second ring transmission line, thereby constructing a transmission pass bypassing the failure point. The method for notifying the port number to each port has been described, for example, in the Japanese Patent Unexamined Publication No. 58-117750 (corresponding to U.S. Pat. No. 4,510,493).
However, in the prior art abnormality diagnosis method described above, if another failure occurs on the input side of the second ST in the line concentrator after the failure of the ring transmission line for example, the control ST receives an abnormality notification frame from the second ST and thus initiates an operation to establish the loopback path, which causes the bypass operation of the failure position in the line concentrator to be delayed. Similarly, when a failure occurs in a branch line on the output side of an ST, an ST at a location downstream with respect to the ST and adjacent thereto and an ST next to the downstream ST are considered to correspond to the first and second ST's, respectively. Under this condition, the bypass operation is executed by the control ST as described above. As a consequence, the bypass operation of the ST on which the failure has actually occurred is delayed.
In addition, since this abnormality diagnosis method is implemented under conditions that each ST has an information of a port number for its connection and issues a frame containing the port number when an abnormality is detected, this method is not suitable for a network allowing ST's to be arbitrarily assigned to free ports of a line concentrator.
Other prior art techniques have been described, for example, in the Japanese Patent Unexamined Publication Nos. 57-135544, 57-135545, and 57-135546 in which an internal ST of each line concentrator issues a diagnosis frame to the spare line, namely, the second ring transmission line and the state of self-concentrator is diagnosed by receiving the diagnosis frame. However, these publications have not considered a case in which a failure occurs on the second ring transmission line, which disables an internal diagnosis of a line concentration when an abnormality is present on the second ring transmission line.